Field Effect transistors (FETs) and other semiconductor devices are much used as high frequency amplifiers. A non-limiting example of semiconductor device materials suitable for such applications is III-V semiconductor material, as for example, GaAs. Various types of semiconductor devices including FETs can be constructed in this and other semiconductor materials. Other non-limiting examples of semiconductor devices are MESFET, HEMT, PHEMT, MOSFET, JFET and Bipolar devices.
With FETs, it is often the case that such devices are depletion mode devices, that is, in the absence of applied bias, they are in a conductive or "ON" state and bias must be applied to the control terminal to control their conductance and turn them OFF. When they are operated as linear or quasi-linear amplifiers, especially when full drain swing is needed, it is usually necessary to provide a bias to the control terminal in order to ensure proper operation. A further difficulty is that when a voltage of a given polarity (e.g., positive) is to be applied to the device to provide power, the control bias required for proper operation must often be of the opposite polarity (e.g., negative). Thus, there is frequently a need to provide a control terminal bias supply having the opposite polarity than the main circuit supply. For convenience of explanation in describing the construction and operation of such amplifiers, and not intended to be limiting, it is assumed that the main circuit supply is positive and the bias supply is negative. Persons of skill in the art will understand that other choices of main circuit supply and bias supply polarity can also be made depending upon the types of devices being employed.
In many portable applications it is desired to operate from a single power supply. In this situation, if the main circuit supply is positive and the bias supply is negative, then the negative bias potential must be generated from the positive potential. In the prior art this has conventionally been done using charge pump system or an oscillator and rectifier arranged to produce a negative output. Unfortunately, these prior art charge pumps or oscillator-rectifier supplies have been relatively bulky and difficult or impossible to integrate, especially, on the same integrated circuit chip or other monolithic assembly as the amplifiers themselves. Thus, there continues to be a need for an improved negative bias supply for control of such high frequency amplifiers.